Concern over the possible depletion of fossil fuel energy sources has generated intense interest in recent years in the search for and development of alternative energy sources. Contemplated alternative energy sources include solar energy utilized as electricity either directly through photovoltaic devices or indirectly through thermal devices. The latter has not received as much attention as the former which will, as presently contemplated, use semiconductor devices. These devices are presently relatively expensive power sources, compared to fossil fuel devices, because the devices collect light generally in proportion to the area of the photosensitive junction which must be large to generate useful photocurrents. The cost of manufacturing such devices depends mainly upon the area of the photosensitive junction and is presently too high to permit successful commercial exploitation in other than specialized applications.
Considerable effort has therefore been expended in attempting to find ways to reduce the cost of solar energy obtained from semiconductor devices. One approach that has generated much interest and enthusiasm recently is a liquid-semiconductor solar cell in which the active part of the cell is a junction formed at a liquid-solid interface. The characteristics of this type of cell are discussed by Gerischer in The Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 58, 263 (1975). The junctions in these devices promise to be less costly to manufacture than are the junctions in devices in which the junction is formed between two solids as relatively costly epitaxy or diffusion procedures are not required to form the junction which forms spontaneously in these devices at the semiconductor-liquid interface.
Many semiconductors are presently candidates for the electrode material in such cells. One of the most promising materials is GaAs. GaAs has a bandgap of about 1.4 ev and, since a bandgap of approximately this magnitude theoretically will give the most efficient photovoltaic conversion of solar power into electricity, a high efficiency cell using this material would be extremely desirable from a commercial point of view.
Operation of semiconductor liquid junction photocells using GaAs electrodes and producing stable photocurrent output over an extended time period has been reported in Science 196 1097 (1977). The cells reported used n-type single crystal GaAs electrodes in an electrolyte comprising a redox couple consisting of selenide/polyselenide anions. At air mass two (AM2) illumination, open circuit voltages were between 0.65 and 0.75 volts; short circuit currents were between 14 and 18 ma/cm.sup.2 ; and the fill factors were between 0.55 and 0.72. The reported efficiency at AM2 for conversion of solar energy into electricity was approximately 9 percent.